Wind machines are commonly used to prevent frost damage to vulnerable crops. Frost is often caused by a temperature inversion which produces a cold layer of atmosphere, often not more than fifteen feet deep, that hovers close to the ground. Thus, by circulating the warmer air from above the inversion down and through the crop, the frost can often be prevented and the crop saved. In citrus orchards, where one heavy frost can cause severe crop and tree damage, permanent wind machines are often positioned throughout the orchard. These machines typically consist of a single propeller mounted on a tall pole, which is permanently mounted in the ground at its base. The propeller may be driven either by an electric motor or a combustion engine, and the pole is sufficiently tall to ensure that the propeller will be above the inversion layer and able to circulate the warmer upper air effectively.
While such permanent wind machines are useful for crops such as citrus fruit where the threat of frost is an ongoing one, they are prohibitively expensive for other types of crops which are only vulnerable to an unusual late Spring or early Fall frost. They are also impractical for performing other useful functions of wind machines, such as drying crops like strawberries, grapes, onions, and other row crops which can be damaged by a late Spring rain, or for drying other surfaces which have been soaked by a heavy rain or dew, such as athletic fields, race tracks, golf courses, and streets which are to be paved. For such applications, a portable wind machine is needed, which can be moved to the desired locale to complete a task and then moved to another site.
One type of prior art portable wind machine includes a propeller-type fan which is mounted on a trailer or the like on a low stand. One such machine is shown, for example, in Neundorfer U.S. Pat. No. 2,602,267. Because of its low height, it is easily transported, but is not as effective in preventing frost damage, since it is incapable of circulating warm upper level air. Furthermore, it is not effective over as large a ground area because of its lack of height. An additional drawback is the danger of injury because of the easily accessible fan blades positioned so close to the ground.
A second type of machine is a propeller-type fan which is mounted on a trailer or the like on a taller stand. This type is more effective for preventing frost and for drying large sectors of fields or streets, but if made sufficiently tall is difficult to transport. If it is tall enough for its intended purpose, it may be too tall to fit under overpasses and makes the handling characteristics of the towing vehicle much worse because of a great deal of wind resistance.